Boris Johnson appeared to compare Nick Clegg to a condom this morning in his latest attack on the Deputy Prime Minister.

The Mayor of London said the Liberal Democrat leader was a "prophylactic protection device" to shield Prime Minister David Cameron when he makes decisions that "cheese" people off.

The two men, who both take part in regular phone-in shows on LBC 97.3 have repeatedly taken potshots at one another in the past, with Mr Clegg branding the mayor a ''slacker'' for only hosting a monthly call-in on the station while Mr Johnson claimed the DPM was an " idle bum''.

Mr Johnson was pressed about his views on Mr Clegg's criticism yesterday following Chancellor George Osborne's announcement that £12 billion of new cuts to benefits will be needed following next year's general election.

He told LBC 97.3: "I don't want to get into some sort of endless ding dong with poor old Cleggers.

"He's there to fulfil a very important ceremonial function as David Cameron's lapdog come prophylactic protection device for all the difficult things that David Cameron has to do that cheese off the rest of the...."

Presenter Nick Ferrari interjected saying he had "never heard it called that before" prompting the Mayor to reply "you know what I mean".

Mr Ferrari said: "Actually, I don't know what you mean and I don't think I want to hear what you mean in that particular.."

Mr Johnson said: "He's a kind of shield, he's a lapdog who has been skinned and turned into a shield to protect the...."

A source close to Mr Clegg said: "Boris needs to make up his mind. Half the time he's moaning that we are blocking policies and half the time we're a lapdog.

"All he's done for the people of London is put his name to Ken Livingstone's bike plan and bang on about a fantasy airport.